Return more data than model contains using Spring Data - spring-data

Question

I'm working with Spring Data which is great stuff, but sometimes I need to get more data from database than my model can handle. For example I have model like below.
#Entity
#Table(name = "email")
public class Mail implements Serializable {
#Getter
#Setter
#GeneratedValue(strategy = GenerationType.IDENTITY)
#Id
private Long id;
#Getter
#Setter
private String text;
}
An I my query will be more complex than usual. I want to get my model and in addition number of similar entities, using group by.
#Query(value = "SELECT m, COUNT(m) as countValue FROM Mail m GROUP BY m.text")
List<Mail> findAllNewsletters();
How I should handle something like that? My model does't contain countValue so I will get List<Object[]>
How to deal with that situation, keep my code clean, easiness
of using this.

Answer 1

Step 1: Create a container class to hold the output from your query.
class MailOccurence {
private final Mail mail;
private final Long recurrence;
public MailOccurence(final Mail mail, final Long recurrence) {
this.mail = mail;
this.recurrence = recurrence;
}
public Mail getMail() { return mail; }
public Long getRecurrence() { return recurrence; }
}
Step 2: Populate and return instances of the container class from the query.
Query(value = "SELECT new MailOccurence(m, COUNT(m)) FROM Mail m GROUP BY m.text")
List<MailGroup> findAllNewsletters();
For full details, see the JPA specification.

Answer 2

You can go for a DTO like following
public class MailEntry {
private Long id;
private String text;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getText() {
return text;
}
public void setText(String text) {
this.text = text;
}
}
and inside your business logic you can take the advantage of spring template something like following
#Autowired
JdbcTemplate jdbcTemplate;
private static final String SQL = "SELECT m, COUNT(m) as countValue FROM Mail m GROUP BY m.text";
public List<MailEntry> getMailEntries() {
List<MailEntry> mailEntryList = jdbcTemplate.query(SQL, new RowMapper<MailEntry>() {
public MailEntry mapRow(ResultSet rs, int rowNum) throws SQLException {
MailEntry mailEntry = new MailEntry();
mailEntry.setId(rs.getInt(1));
mailEntry.setText(rs.getString(2));
return mailEntry;
}
});
return mailEntryList;
}
Hope this help.